In the largest study published to date using immunotherapy to treat lung cancer, the drug Keytruda (pembrolizumab) was tested on approximately 500 patients with non-small cell lung cancer.

Because so many of the patients in the study showed significant long-lasting responses, in October 2014 the US Food and Drug Administration (FDA) granted the drug "breakthrough therapy" status for use in lung cancer, allowing it to be fast-tracked for approval.

"The approval of this drug and a test to identifypatients most likely to benefit has the potential to transform the way that lung cancer is treated," said Dr Edward Garon, the study's principal investigator and a researcher at University of California, Los Angeles' Jonsson Comprehensive Cancer Center.

The response rate and duration of response for Keytruda were much greater than for drugs traditionally used to treat lung cancer.

Keytruda, a drug that has already been proven to extend the lives of people with advanced melanoma, is an antibody that targets the protein PD-1, which is expressed by immune cells.

When it binds to another protein called PD-L1, PD-1 acts as an immune checkpoint, dampening the immune system's T cells which otherwise could attack cancer cells, said Garon.

The study was the first to validate that clinical outcomes with this class of drug were directly associated with the level of PD-L1 expression.